1. Field of the Invention
The present invention relates generally to a video display appliance, and more particularly, to an apparatus for preventing an auto-convergence error in a projection television receiver (TV) which prevents generation of an error when an auto-convergence is performed.
2. Description of the Related Art
Generally, diverse types of video display appliances including a small-sized analog television receiver (TV) to a large-sized projection TV of more than 60 inches have been provided to consumers.
A conventional apparatus for transmitting/receiving digital video data, as shown in FIG. 1, includes a transmitter part 100 which includes a video processor 102 and a digital-to-analog (DA) converter 104, and a receiver part 106 which includes an analog-to-digital (AD) converter 108 and a video processor 110.
A projection TV is an appliance which displays an image on a screen by projecting R, G, B colors on the screen using R, G, B Braun tubes, and its sensitivity items are W/U (White Uniformity), B/U (Bright Uniformity), convergence, focus, distortion, etc.
The convergence is to gather R (Red), G (Green), B (Blue) beams emitted from electron guns onto one spot on a screen by a magnetic field of a deflection yoke. If the electron beams are not properly deflected due to an abnormal operation of the deflection yoke or an influence of a magnetic field, a mis-convergence is generated, which deteriorates the color balance. That is, if the convergence is normally performed and the R, G, B beams accurately converge on a spot, an image of a white color is displayed. If a mis-convergence is generated, lines having colors of R, G, B may appear near by the image of the white color, and this deteriorates the picture quality.
Accordingly, in order to clearly display the image of the white color, the R, G, B beams should be gathered onto one spot, which is called an auto-convergence.
According to this auto-convergence, a specific pattern is produced on the screen, the degree of mis-convergence is detected using an optical sensor, and this mis-convergence is collected.
Hereinafter, a conventional convergence control apparatus for a projection TV will be explained with reference to the accompanying drawings.
FIG. 1 is a block diagram illustrating the construction of a conventional convergence control apparatus in a projection TV. FIG. 2 is a view showing a sensing method for an optical sensor according to the movement of a measurement pattern, and FIG. 3 is a waveform diagram illustrating an output waveform of an optical sensor according to the movement of the measurement pattern.
The conventional convergence control apparatus in a projection TV, as shown in FIG. 1, includes a screen 10 for displaying an image signal, first to eighth sensing units 11 to 18 mounted in a specified area of the screen 10, an amplifying unit 20 for receiving and amplifying sensed values outputted from the first to eighth sensing units 11 to 18 with a predetermined amplification factor, a comparing unit 30 for receiving amplified values outputted from the amplifying unit 20 through inverting (−) terminals and non-inverting (+) terminals and comparing the inputted amplified values, an inverter 40 for inverting the output value of the comparing unit 30, a D-type flip-flop 50 for latching an output value of the inverter 40, and a microcomputer 60 for receiving data through the D-type flip-flop 50 and outputting a control signal for a convergence control.
The first, third, fifth and seventh sensing units 11, 13, 15 and 17 among the first to eighth sensing units are positioned in the middle parts of borders of the screen 10, and the second, fourth, sixth and eighth sensing units are positioned at the corners of the screen 10, respectively. Also, the sensing units 11 to 18 are classified into a first group composed of the first to fourth sensing units 11 to 14 and a second group composed of the fifth to eighth sensing units 15 to 18, and each of the first to eighth sensing units 11 to 18 includes first and second optical sensors A and B as shown in FIG. 2.
The amplifying unit 20 includes a first amplifying unit 21 for receiving and amplifying the sensed values outputted through the first optical sensors A and the second optical sensors B of the first to fourth sensing units 11 to 14, and a second amplifying unit 22 for receiving and amplifying the sensed values outputted through the first optical sensors A and the second optical sensors B of the fifth to eighth sensing units 15 to 18.
Here, the first amplifying unit 21 includes a first amplifier 21a for receiving and amplifying values sensed through the first optical sensors A of the first to fourth sensing units 11 to 14, and a second amplifier 21b for receiving and amplifying values sensed through the second optical sensors B of the first to fourth sensing units 11 to 14. The second amplifying unit 22 includes a third amplifier 22a for receiving and amplifying values sensed through the first optical sensors A of the fifth to eighth sensing units 15 to 18, and a fourth amplifier 22b for receiving and amplifying values sensed through the second optical sensors B of the fifth to eighth sensing units 15 to 18.
The comparing unit 30 includes a first comparator 31 for receiving and comparing output values of the first and second amplifiers 21a and 21b, and a second comparator 32 for receiving and comparing output values of the third and fourth amplifiers 22a and 22b. 
The operation of the conventional convergence control apparatus for a projection TV will now be explained.
If an auto-convergence control mode is selected, the position of a pattern is sensed through the first to eighth sensing units 11 to 18. The sensed values outputted through the first optical sensors A of the first to eighth sensing units 11 to 18 are inputted to the second and fourth amplifiers 21b and 22b, and the sensed values outputted through the second optical sensors B of the first to eighth sensing units 11 to 18 are inputted to the first and third amplifiers 21a and 22 the amplification factor of which is twice the amplification factor of the second and fourth amplifiers 21b and 22b. 
The amplified value b1 outputted through the first amplifier 21a is inputted to the inverting (−) terminal of the first comparator 31, and the amplified value al outputted through the second amplifier 21b is inputted to the non-inverting (+) terminal of the first comparator 31, so that a compared value c1 is outputted from the first comparator 31. Also, the amplified value b2 outputted through the third amplifier 22a is inputted to the inverting (−) terminal of the second comparator 32, and the amplified value a2 outputted through the fourth amplifier 22b is inputted to the non-inverting (+) terminal of the second comparator 32, so that a compared value c2 is outputted from the second comparator 31.
The output values of the first and second comparators 31 and 32 are AND-gated, and then inputted to the inverter 40. The inverted value outputted through the inverter 40 is latched in the D-type flip-flop 50, and then inputted to the microcomputer 60.
Here, the sensing method according to the movement of the measurement pattern performed through the first to eighth sensing units 11 and 18 will be explained with reference to FIGS. 2 and 3.
If the pattern moves downwards in the case that the measurement pattern is red (or green or blue), the third sensing unit 13 performs the sensing process. As shown in FIG. 2, when the pattern is at a first sensing point t1, the pattern falls on neither of the first and second optical sensors A and B, and no light is sensed by both the first and second optical sensors A and B. In this case, low-level signals are applied to the first and second amplifiers 21a and 21b, and this causes low-level signals to be outputted from the first and second amplifiers 21a and 12b as shown in FIG. 3.
The low-level signals outputted from the first and second amplifiers 21a and 21b are inputted to the non-inverting (+) terminal and the inverting (−) terminal of the first comparator 31, and the first comparator 31 outputs a high-level signal. This is because several pull-up resistors are connected to the non-inverting (+) terminal of the first comparator 31.
Also, as shown in FIG. 2, if the pattern moves further and reaches a second sensing point t2, ½ of the pattern falls on the first optical sensor A, but no pattern falls on the second optical sensor B and no light is sensed by the second optical sensor B. In this case, the output value of the second amplifier 21b is larger than the output value of the first amplifier 21a, and thus the first comparator 31 continuously outputs a high-level signal.
If the pattern moves further and reaches a third sensing point t3, the pattern falls on the first optical sensor A in full, but ½ of the pattern falls on the second optical sensor B. In this case, the output value of the second amplifier 21b, which has received the sensed value of the first optical sensor A, becomes twice the output value of the first amplifier 21a, which has received the sensed value of the second optical sensor B (i.e., A=2B). However, since the amplification factor of the first amplifier 21a is twice the amplification factor of the second amplifier 21b, the output value of the first amplifier 21a is equal to the output value of the second amplifier 21b, and thus the first comparator 31 outputs a low-level signal thereafter.
In the same manner, the second comparator 32 outputs a low-level signal. The outputs of the first and second comparators 31 and 32 are AND-gated, and this AND-gated signal is inverted into a high-level signal through the inverter 40. This inverted high-level signal is inputted to the D-type flip-flop 50 as its clock so that the D-type flip-flop latches data, and then inputted to the microcomputer 60.
The microcomputer 60 judges the time point where its port goes from a low level to a high level as the third set position t3, judges the degree of the mis-convergence by measuring the distance from the initial position of the pattern to the third set position t3, and outputs a corresponding collection control signal.
The conventional auto-convergence error preventing apparatus in a projection TV, however, has the following problems:
First, if it is assumed that the pattern falls on the second set position t2 while the pattern is moved in order for any one of the optical sensing units mounted on the border of the screen to search for the third set position t3, the output of the comparator must be the high-level signal. However, if a surrounding light is inputted to other optical sensing units, the outputs of the corresponding comparators are varied, and this may cause the microcomputer to misrecognize the set position.
Second, if an external light is inputted to other sensing units besides the sensing unit subject to sensing, it affects the present sensing position, and it is difficult to measure the position accurately.
Third, as it is difficult to measure the position accurately, the convergence cannot be accurately controlled which deteriorates the picture quality.